1. Field of the Invention
The present invention relates to magnetic memories, such as magnetic random access memories (MRAMs), using magnetoresistive elements (hereinafter also referred to as xe2x80x9cmagnetic memory elementsxe2x80x9d) in which data is written by a magnetization direction and data is read by a magnetoresistance effect.
2. Description of the Related Art
Although a MRAM is a solid-state memory with no active parts as in the case of a semiconductor memory, in the MRAM, data is not lost even if a power supply is cut off, rewriting can be performed for an unlimited number of times, and there is no danger that memory contents may disappear due to exposure to radiation, all of which are advantageous in comparison with the semiconductor memory.
As the memory element used for the MRAM, a magnetoresistive element is preferably used, in which an external magnetic field is applied to the magnetic layers while a predetermined current is applied between the magnetic layers, the resistance changes in response to the relative angle between the magnetization directions of both magnetic layers. When the magnetization directions of the magnetic layers are parallel to each other (i.e., the magnetization directions of the magnetic layers are the same), the minimum resistance occurs, and when the magnetization directions are antiparallel to each other (i.e., the magnetization directions of the magnetic layers are opposite to each other), the maximum resistance occurs.
Accordingly, by using the magnetic layers having different coercive forces, the parallel state and the antiparallel state can be brought about in response to the strength of the magnetic field, and thus the magnetization state can be sensed by the change in resistance.
Recently, tunneling magnetoresistive (TMR) elements have been developed, in which a surface-oxidized Al film is used as the tunneling barrier layer sandwiched between two magnetic layers, resulting in a magnetic memory element exhibiting a rate of change in magnetoresistance of approximately 20%. Therefore, it is highly possible to apply the tunneling magnetoresistive element in magnetic heads and magnetic memory elements. Magnetoresistive elements exhibiting such a large rate of change in magnetoresistance have been reported, for example, in the Journal of Applied Physics, vol. 79, 4724 4729, 1996. With respect to memory elements used for magnetic memories, U.S. Pat. No. 6,219,275 discloses a magnetic memory using a magnetoresistive element in which perpendicular magnetization films are used as magnetic films.
In order to write data in a MRAM, currents are passed through lines placed in the vicinity of the individual memory cells to generate magnetic fields, and since the magnetization directions of magnetic layers (memory layers) are determined by the magnetic fields, data is written. Therefore, in order to perform writing, currents which can generate magnetic fields sufficient for reversing the magnetization directions of the memory layers must be passed through the lines. For that purpose, considerably large currents of approximately several to 10 mA are required.
In order to reduce a current to be passed through the lines during writing, for example, U.S. Pat. No. 5,894,447 discloses a configuration in which in-plane magnetization films are used as the magnetic layers, upper and lower write word lines are placed so as to sandwich the magnetic layers, and the ends of the upper and lower write word lines are connected to each other so that the current flows from the upper write word line to the lower write word line in a turn-back manner.
FIGS. 12A to 12C are schematic diagrams showing a configuration of a conventional magnetic memory. FIG. 12A is a sectional view of a memory cell, FIG. 12B is a plan view showing a plurality of memory cells adjacent to each other, and FIG. 12C is a sectional view of a cell array along word lines. FIG. 12C also shows a drive circuit for driving the word lines.
As shown in FIGS. 12A and 12B, in each memory cell 36, bit lines 31 are placed orthogonally to an upper word line 32 and a lower word line 33 formed above and below the bit lines 31, respectively. A giant magneto-resistance (GMR) film 34 is formed at the intersection, both ends of the GMR film 34 being connected to the bit lines 31. That is, the upper word line 32 is formed directly above the GMR film 34, and the lower word line 33 is formed beneath the GMR film 34. The upper word line 32 and the lower word line 33 overlap in the vertical direction with the GMR film 34 and interlayers therebetween. The interlayers electrically insulate the upper word line 32 from the lower word line 33 and electrically insulate the upper and lower word lines 32 and 33 from the GMR film 34 and the bit lines 31.
When a current is passed through the upper word line 32 of the memory cell 36, for example, toward the front as shown in FIG. 12A by a circular mark having a dot therein, and a current is passed through the lower word line 33 toward the back as shown in FIG. 12A by a circular mark having a cross therein, both magnetic fields generated by the currents flowing through the upper word line 32 and the lower word line 33 are directed rightward in the drawing in accordance with the Ampere""s corkscrew rule. As a result, a combined magnetic field is produced by combining the magnetic fields generated by the currents flowing through the upper word line 32 and the lower word line 33, and the combined magnetic field is applied to the GMR film 34. The combined magnetic field applied to the GMR film 34 has a magnetic intensity approximately twice the intensity of the magnetic field generated by one word line on the assumption that the magnitude of the current supplied is the same.
For example, as shown in FIG. 12C, in a memory cell array including memory cells 36a to 36l, the upper word line 32 and the lower word line 33 are extended from the memory cell 36a on the extreme left to the memory cell 36l on the extreme right, and the upper word line 32 and the lower word line 33 are connected in series by a contact 37 at the left edge of the cell array. Furthermore, the right ends of the upper and lower word lines 32 and 33 are connected to a drive circuit 35, and a current is supplied in the directions indicated by the arrows in the drawing by applying voltages V1 and V2 thereto.
As a result, at the same current consumption as in the case of a traditional magnetic memory, the combined magnetic field having a magnetic intensity approximately twice the magnetic intensity in the traditional magnetic memory can be produced by the current flowing through both word lines.
However, in the configuration which uses the in-plane magnetization films as the magnetic layers and in which word lines are provided so as to sandwich the memory cells as described above, the fabrication process is difficult, for example, because the upper lines and the lower lines must be connected to each other at the ends by through holes or the like. Additionally, parasitic capacitance occurs between the upper word lines and the lower word lines, which may give rise to a problem, in particular, when high-speed driving is performed. Moreover, because of the multilayered structure, the aspect ratio (x/y shown in the drawing) of the lines tends to increase, and therefore, it is difficult to decrease the opposing areas of the upper and lower word lines which are directly related to parasitic capacitance.
Additionally, when the in-plane magnetization films are used for memory cells, it is difficult to miniaturize the memory elements under the influence of curling of magnetization, etc. In order to solve this problem, the configuration in which perpendicular magnetization films are used as the magnetic layers as disclosed in U.S. Pat. No. 6,219,275 may be employed. However, since the intensity of magnetic fields for reversing the magnetization is increased as memory elements are further miniaturized in the future, large currents are needed particularly for reversing the magnetization in the data-writing process, resulting in a difficulty in saving power. In such a case, when a plurality of write lines are provided for each memory element in order to apply a large magnetic field to one memory element, since a drive circuit is connected to each write line, the number of drive circuits is increased and the area of the circuits in the periphery of the memory is increased.
It is an object of the present invention to provide a magnetic memory in which data is retained stably even if the memory element is miniaturized, which can be operated with low power usage, and which can be fabricated by a simple fabrication process.
It is another object of the present invention to reduce the area of the circuits in the periphery of the memory.
In one aspect of the present invention, a solid-state magnetic memory includes: a substrate; a plurality of memory cells arrayed in a matrix on the substrate, each memory cell including a memory element and an element-selecting device, the memory element including two magnetic layers and a nonmagnetic layer sandwiched between the magnetic layers, the easy magnetization axis of each magnetic layer being directed perpendicular to the plane of the layer; a plurality of bit lines connected to the memory elements for reading out data recorded in the memory elements; and a plurality of write lines placed substantially flush so as to sandwich the memory cell columns, at least one end of each write line being joined to one end of another write line so that a current flows in opposite directions at both sides of each memory cell column.
In another aspect of the present invention, a method for making the solid-state magnetic memory includes the step of forming the write lines by patterning using the same mask so that the write lines are formed substantially flush. Consequently, the write lines and the joints thereof can be formed by one process.
In another aspect of the present invention, a solid-state magnetic memory includes a plurality of memory cells arrayed in a matrix, each memory cell including a memory element composed of a magnetoresistive element and an element-selecting device, the magnetoresistive element including two magnetic layers and a nonmagnetic layer sandwiched between the magnetic layers, the easy magnetization axis of each magnetic layer being directed perpendicular to the plane of the layer, in which write lines shared by two adjacent memory element columns are placed, both ends of the shared write lines being joined to the ends of other write lines so that a current flows with two memory element columns therebetween, the other ends of the other write lines being joined to a drive circuit and a power supply circuit, and thereby two adjacent memory elements in the memory element columns placed at both sides of the shared write line are complementarily operated to write one bit of data.
In another aspect of the present invention, an information device includes the solid-state magnetic memory as a built-in memory.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.